No. 6. DEPARTMENT OF AGRICULTURE. 677 



general idea of the experiments is to keep a sort of debit and ci-edit 

 account with the auinuil. The animal is charged with the potential 

 energy of the food which it consumes; this is measured by burning 

 a small sample of the food uuder such conditions that all the heat 

 produced can be collected and measured; this heat gives us the meas- 

 ure of the total potential energy of the food. On the other side of 

 the account, the animal is to be credited first, with the potential 

 energy of those portions of the food which are not burned up in the 

 body, but pass into the excreta. This corresponds to the mud 

 filtered out of the water in our illustration of the reservoir or it may 

 be compared to the ashes and flue gases of the furnace in which coal 

 is burned. Second, the animal is credited with the heat which it 

 gives off. It is this heat which it is one of the prime purposes of the 

 respiration-calorimeter to measure and the most complicated and 

 delicate portion of the apparatus is that devoted to the accurate 

 measurement of the amount of heat produced by the animal. Third, 

 the animal is credited with the energy of any work which it may per- 

 form, although in our own experiments, this factor we will for the 

 present not enter in. Comparing now the amounts on the two sides 

 of the account, if the debit side is larger than the credit; that is, if 

 the animal has received more energy than it has given off, there has 

 been a storage of energy in the form of a gain of flesh or fat which 

 should be entered on the credit side of the account to balance it, 

 while, on the other hand, if the credit side is the larger, there has 

 been a loss in the energy of the body showing that the animal has 

 been living in part on its own tissues. 



The amount of energy gained by the animal upon a given food 

 gives us the clue to the determination of the availability of the 

 energy ci' that food. Suppose, for example, that we put a steer upon 

 a ratioii which is exactly sullficient to maintain it; that is, which sup- 

 plies just enough available energy to keep up the internal work of 

 the body, and upon which there is consequently no gain or loss of 

 tissue or of energy by the body — in which the inflow to the reservoir 

 exactly equals the outflow. Now, suppose we add to this ration 

 enough of the particular feeding stuff under experiment to furnish 

 to the body 1,()U0 units of energy. Suppose further that our trial 

 iQ the respiration-calorimeter shows us that out of these 1,000 units 

 of added energy, 600 units are added to the heat production of the 

 body and 400 units stored up in the form of gain of fat and flesh. 

 It is evident that the availability of this particular feeding stuff, un- 

 der the conditions of the experiments, is 40 per cent, that is, 40 per 

 cent, of its energy can be stored up in the body while GO per cent, is 

 consumed io the work of digesting and assimilating the food and sim- 

 ply serves to increase the heat given off from the body. Let us sup- 

 pose now that we repeat the experiment with another feeding stuff. 



